Automobiles are equipped with restraint systems, such as seat belts, and inflatable restraint systems, such as airbags, to secure safety for passengers. However, in some situations, these safety devices could actually injure the occupants. For example, an occupant of the front passenger seat may be injured by the deployment of an airbag if the occupant is a baby or child. In recent years, there is a trend for controlling the operation of such safety devices according to the weight of a passenger for improved performance of seat belts and airbags. For example, the amount of gas introduced into the airbag, an airbag inflating speed, or a pre-tension of the seat belt are adjusted according to the weight of a passenger. For that purpose, it is desirable to provide a device for measuring the weight of a passenger sitting on a vehicle seat and preventing or modifying the actuation of the airbag when the weight is less than a predetermined amount.
There have been a number of proposals for the measurement of occupant weight, all with significant disadvantages. One known device provides a gel-filled cushion integrally attached to the vehicle seat, as described in U.S. Pat. No. 6,041,658. The pressure on the liquid in the cushion is measured to determine the occupant's weight. The disadvantage of this known device is that the accuracy of the weight measurement varies with the position of the occupant and the angle of the vehicle seat back. Furthermore, the puncture-resistance feature of the gel-filled bag is not perfect. The gel-filled bags often have a sealing problem or get punctured, resulting in gel leakage from the bag. If the leakage is undetected, the weight measurement result will be inaccurate. Accordingly, it is desirable to provide a reliable weight measurement system that provides an accurate measurement regardless of the seated position of the occupant or the position of the vehicle seat back.
Another known measurement system includes the use of force sensors at a plurality of support points on the seat. Typically, four load cells are used, each located at one of the four corners of the seat, between the seat and the vehicle floor. The disadvantage of this system is that each sensor significantly increases the cost of the weight detection system. Furthermore, depending on the seat configuration and the occupant position, the weight measured by each sensor could vary, resulting in a wide range of force difference between the sensors. Generally, electronics are used to average the signals received by the sensors to determine an average weight sensed by the four sensors. However, the averaging process introduces further errors in the measurement. Also, the load cells themselves can be damages by crash forces or other overloads.
Accordingly, it is desirable to provide an accurate weight measurement system that is also cost-efficient, easy to install and repair. It is further desirable to provide a system that provides failure detection capability that alerts the user of a failure and thus reduces the risk of incorrect information provided to a safety restraint system. In the event that the vehicle floor or seat becomes distorted, a wracking load may be placed on the load sensors, and the unequal loading may result in inaccurate weight measurement. Accordingly, it is desirably to provide a system that can transfer wracking loads effectively to avoid inaccurate measurement of the passenger's weight. Finally, a weight measurement system that protects the sensor itself from overloads is highly desirable.